Planet hunters find exoplanet that could potentially support life

NASA has discovered what appears the first Earth-like planet orbiting at a …

Yesterday, NASA and the NSF announced the detection of what may be the first Earth-like planet orbiting in its star's habitable zone, meaning that it might play host to liquid water. This is a pretty big deal. The paper describing the findings calls this "one of the holy grails of exoplanet research," and one of the scientists involved in the discovery, in a fit of over-enthusiasm, even claimed that he was certain there would be life there. But there's actually no guarantee that, even within the habitable zone, liquid water will be present, much less life. That's the bad news, though; the good news is that its discovery implies that similar planets might be relatively abundant.

The star, Gliese 581, is only about 20 light years away. The team behind the new exoplanet reasoned it was a good candidate for observations because it's relatively small and cool (an M-class dwarf). The low mass makes the influence of orbiting planets easier to detect, while its cool temperature means that the habitable zone is relatively close to the star—planets in that region are easier to detect. So, about 10 years ago, the team started gathering observations of Gliese 581 and a few hudred similar stars using the Keck Observatory in Hawaii.

Gliese 581 was already known to host several planets, and some of these have already been suggested as potentially habitable—in fact, the paper refers to it as having "a somewhat checkered history of habitable planet claims." The outer of this pair of planets is probably too cold, while the inner one is only cool enough if you assume the greenhouse effect doesn't exist (which, for the record, no one does, so it is now considered too hot for liquid water).

A relatively Earth-like planet

But, with a dozen years of observation, the authors claim to have data indicating two new planets, the fifth and sixth in the Gliese 581 system. One of those, GJ 581f, is big (at least seven times the Earth's mass) and orbits well beyond the area that's warm enough to support liquid water. But GJ 581g is only about three times the mass of Earth, and appears to orbit within the habitable zone. In the case of Gliese 581, that means a distance of 0.146 AU and an orbital period of 36.6 days.

The planet wasn't imaged directly. Instead, the authors used the traditional system of looking for periodic changes in the star's spectrum, caused by the pull of planets as they orbit. These show up as Doppler shifts in the light we can observe from Earth. A substantial portion of the paper is spent on describing how systemic sources of noise are limited in order to make the signals apparent. For example, during imaging, they placed an iodine cell in front of the spectroscope slit in order to use the absorption typical of that element as a scale for identifying small shifts in the light from the star.

In fact, the team had tried to submit a paper last year, but it got shot down during peer review because of potential sources of noise. "One of the referees (from the HARPS team)," the new paper acknowledges, "kindly raised the concern (based partly on our larger value for apparent stellar jitter) that we may have some residual systematics that could be affecting the reliability of some of our conclusions." So, the authors got another year of data and redid their entire analysis.

With 11 years of spectral wobbles to work with, the authors set about identifying signals from individual planets, and then subtracting those from the total spectrum. By the time no orbital signals were left, they had identified six planets, including the two new ones. All of these fit comfortably inside Earth's orbit; five of them orbit closer than Mercury. The data indicates that the orbits are roughly circular, and modeling confirms that the system would be stable on reasonable time scales.

Although this revealed two new planets, the attention is on GJ 581g, since, despite its larger mass, a bigger radius estimate places its surface gravity very close to Earth's, at between 1.1 and 1.7g. Based on the output of the star, its surface temperature is estimated at 228K. Assuming an Earth-like greenhouse effect, that temperature rises to between 236K and 258K, a bit below the Earth's 288K. But this wouldn't be evenly distributed, because the planet is tidally locked to its host star, meaning that it always faces a single side towards it. Thus, there may be different temperature zones at the interface between the planet's light and dark sides.

It's important to emphasize that the assumption of an Earth-like greenhouse is critical for getting this planet into a zone where liquid water could exist. A weaker or stronger one could easily push the planet out of the zone. It also says nothing about the planet's geology and chemistry, which could easily lock most of the water in rocks or turn it into an acidic soup that stretches the definition of habitable. The scientist who was certain of the presence of life was enthused because life on Earth flourishes in even these harsh environments (frozen, dry, and acidic), but that says little about whether life could arise under these conditions.

In any case we aren't likely to know more until we have hardware that can image the planet's atmosphere directly.

How many more like this?

A later section of the paper considers what this says about the general presence of habitable planets. To a certain extent, the authors cheat by including our own solar system as host to a planet (after all, if it weren't, we wouldn't be here to find GJ 581g). But simply going out to the planet's distance means that 1.7 percent of the stars within that volume host potentially habitable planets.

When you consider only a fraction of the stars would have planets orbiting within a plane that makes it possible to detect them, that number goes up. It goes up further once the fact that we've not been observing most of them long enough to get the sort of data that we have for Gliese 581. By the time the authors are done adding caveats, the number is over 20 percent, and they conclude, "if the local stellar neighborhood is a representative sample of the galaxy as a whole, our Milky Way could be teeming with potentially habitable planets."